Fax Voip T.38 Modem supports receiving and sending faxes according to T.38 standard, as well as in audio mode (over G711-ulaw or G711-Alaw codecs).
Both T.38 and audio fax (fax over G.711 codecs) have their advantages and disadvantages when sending/receiving faxes over VoIP.
T.38 - the most reliable standard used to transmit faxes over VoIP in real time. Isolates from the delays, timing jitter, and packet loss experienced in VoIP networks. T.38 support should be provided by your SIP or H.323 VoIP Service provider. Note that T.38 standard is fully compatible with using any of audio codecs (G.711 codecs are not required).
Sending a fax over the audio channel (G711 fax) is not so reliable. Nevertheless, this method can be used with almost any VoIP provider that supports G.711-ulaw or G.711-Alaw codecs. In this case, possibility and quality of faxing depends on many factors, such as the quality of VoIP termination or on the stability of the Internet channel on the client side. Despite the obvious disadvantages, G711 fax often operates very well and stable with many SIP and H.323 providers.
One of the common problems faced by customers is the incomplete support for T.38 on the side of VoIP Service provider. In other words, the provider announces T.38 support, but can not provide T.38 for some call destinations. Or does not have a stable termination to your desired destination. Fax Voip T.38 Modem is able to automatically detect the presence or absence of T.38 support at the time of the current call.
In the case of sending or receiving faxes via ISDN CAPI 2.0 line using G711 audio fax mode, you should not expect problems as there are no delays typical for VoIP.
Note that both T38 fax mode and G711 fax mode are fully compatible with Voice Modem Functionality.